Problem: Simplify the following expression: $r = \dfrac{a + 11}{4a} - \dfrac{2a + 9}{4a}$
Explanation: Since the expressions have the same denominator we simply subtract the numerators: $r = \dfrac{a + 11 - (2a + 9)}{4a}$ Distribute the negative sign: $r = \dfrac{a + 11 - 2a - 9}{4a}$ Combine like terms: $r = \dfrac{-a + 2}{4a}$